Dry weather impacting mosquito viruses

The peak weeks of mosquito season have yet to arrive, but there are signs the ongoing drought is having an effect on the disease-carrying insects.

“We had very dry conditions last year as well,” said Dr. Catherine Brown, the state public health veterinarian for Massachusetts. “Part the fall was dry, which means there weren’t good water accumulations in the swamps where EEE-carrying mosquitoes like to breed, so they laid small numbers of eggs.”

The impact of the weather on West Nile virus could play out differently.

“With West Nile virus, it’s a little more complicated. It is generally true that hot and dry conditions are supportive of mosquito populations that tend to be associated with West Nile virus,” Brown said.

The last human case of EEE in Massachusetts was in 2013. In 2012, there were seven cases.

“That was also a very hot, dry year,” Brown said. “But with 2012, the difference was that in the fall of 2011, there was plenty of water in the swamps where EEE-carrying mosquitoes breed. If there’s no water in the fall to begin with, like this year, there’s nowhere for the mosquitoes to lay their eggs.”

Public health officials, though, caution that August and September are when EEE and West Nile virus are typically most prevalent.

Lindle Willnow, Hamilton board of health chairman, said mosquito control hasn’t been as pressing an issue in years past due to the drought.

Still, he warned residents an ounce of preparation is equal to a pound of cure.

“You’re better of being salfe and using mosquito repellant,” he said/

This year, Massachusetts first detected West Nile virus in mosquito samples collected July 1 in Worcester.

“We had our first positive early in July, but interestingly enough, it remained lower than what we’d typically expect with hot, dry conditions,” Brown said.

West Nile virus, she explained, is typically brought to Massachusetts by infected migratory birds. Mosquitoes then spread the virus to other birds, and occasionally to humans.

Variations of the virus in bird populations are more difficult to track, but likely influence its prevalence in mosquitoes.

“We’re only able to measure the virus in mosquitoes.” Brown said. “We’re not really able to measure it in birds.”

Priscilla Matton, superintendent of Bristol County Mosquito Control, said that in hot, dry conditions, it can be particularly important to frequently empty out containers of standing water, such as bird baths – which can attract both West Nile virus-carrying birds and the mosquitoes that can spread the disease.

“West Nile virus does do better in a hot, dry summer, so we could potentially see an increase in West Nile virus coming if the weather conditions stay the same,” Matton said.

It’s difficult to predict what the rest of the summer will bring, she said.

“If we were to get a significant amount rain, we could definitely see a much different outcome,” she said. “It is so rain-driven. It is so weather-dependent that it is hard to predict.”

Ellen Bizlack, an entomologist with Plymouth County Mosquito Control, said that culex mosquitoes, which can carry West Nile virus, are the only species that’s doing well this summer.

Her county is often the epicenter for EEE in Massachusetts.

The species of mosquito that typically carries EEE, she said, typically struggles when the water table is low. Despite the lower mosquito population this summer, she points to the positive test result from Middleboro as a sign that EEE is present.

“It is still around even though the normal vectors for that disease are lower,” she said. “Even though the mosquito numbers low, people should still avoid mosquito bites, use repellants and keep an eye on containers in their yard and dump out standing water.”

Another mosquito-borne virus, Zika, has been widespread throughout parts of South America, but is not considered a major risk in Massachusetts. It’s carried by a species of mosquito that has never been found in the Bay State, Brown said.

“It’s considered extremely unlikely that we would see mosquito-borne transmissions of Zika here in Massachusetts,” Brown said.

The Zika virus causes a serious birth defect called microcephaly, putting women who are pregnant or may become pregnant at particular risk. Microcephaly is characterized by an abnormally small head. It can cause developmental delays, intellectual disability and abnormal muscle functionality.

“We would want women who are pregnant or may become pregnant to really think seriously if they want to travel to areas where Zika is present,” Brown said.